The predatory mite Phytoseiulus persimilis is a very efficient biological control agent against the pest Tetranychus urticae, a phytophagous mite. However, when the atmospheric relative humidity is low, P. persimilis struggles to control this pest. My goal is to understand why the efficiency of this predator is decreased in dry conditions, to be able to solve this problem.

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ESR update - My first hypothesis to explain the bad performance of P. persimilis in dry conditions was based on the fact that P. persimilis eggs are sensitive to low humidity. It has been proved already that, unlike the eggs, the mobile life stages (larva, nymph and adult) can compensate the loss of water by drinking or feeding on prey. Based on this information, I assumed that very few P. persimilis eggs would hatch in dry conditions, leading to a low population growth of this predator. To check this hypothesis, I exposed many eggs of P. persimilis to constant low humidity conditions during their full developmental time. Very few of them hatched, and those who hatched developed into larvae and nymphs without problem. I could conclude from this observation that, indeed, P. persimilis eggs are a very sensitive life stage to low humidity. But I still could not confirm my hypothesis : Is it really what is happening in the field? Are P. persimilis eggs really exposed to a constant low relative humidity during their full developmental time? If not, what happens with the egg hatching rate?

To answer these questions, I am trying to simulate more realistic humidity conditions in a second experiment : taking into account the variations in humidity between day and night, and between different days. In the field, it is highly probable that an egg laid on the surface of a leaf will spend at least a few hours under high humidity conditions. In this experiment, instead of being constantly low, the relative humidity is changing during the development of the eggs.

Another important question I want to answer is : are P. persimilis eggs the only reason why this predator is not efficient in dry conditions? My second hypothesis concerning the bad performance of P. persimilis in dry conditions focuses on another life stage : I have indications that P. persimilis adult females may change their oviposition behavior when they are exposed to a low relative humidity. I am currently testing this hypothesis in the laboratory.